It is deemed desirable to employ Fourier transform correlation technology for very high resolution positioning and alignment of various components such as silicon wafers or mask components during the manufacturing process.
The lensless joint transform correlator (LJTC) of the present invention is a new device for optically determining the relative x-y position of two pinholes which can be positioning fiducials. Its operation is similar to the operation of the chirp-encoded and single-lens joint transform correlator of U.S. Pat. No. 5,648,872 issued to Thomas Grycewicz on Jul. 15, 1997 and incorporated by reference herein. As in all variants on the joint transform correlator (JTC), the LJTC is a two stage device. The first stage produces a joint image transform, which is captured by a camera or optically addressed spatial light modulator (OASLM) and retransmitted in a second stage to form the correlation peak. As with the single-lens joint transform correlator, the LJTC forms a Fresnel zone in the transform plane and is easily operated with a large magnification in the output plane. Through binarization of the transform plane and use of a third or fifth order focus of the Fresnel zone, very high output resolution can be achieved. In the LJTC the transform plane is formed by a Fraunhaufer difiraction pattern, and the output is formed by Fresnel diffraction. A limitation of the LJTC is that the inputs must be pinholes or their equivalents such as tiny mirrors, while the closely related single-lens JTC correlates arbitrary input images. However, this limitation is advantageously employed in the present invention to provide a simple displacement measuring system for producing a very clean output signal. For applications like industrial positioning systems, where a pinhole can serve as well as any other alignment mark, the LJTC has the potential to be used in a very fast, high accuracy position measurement system and thus is a good candidate for processes where fast precision alignment is of great importance, such as mask alignment in semiconductor manufacture.